What's the Use? asks why there is such a vast gulf between public perceptions of mathematics and reality. It shows how mathematics is vital, often in surprising ways, behind the scenes of daily life. How politicians pick their voters. How an absurd little puzzle solved 300 years ago leads to efficient methods for kidney transplants. How an Irish mathematician's obsession with a new number system improves special effects in movies and computer games. How SatNav relies on at least six mathematical techniques. And how a bizarre, infinitely wiggly curve, helps to optimise deliveries to your door.